The invention relates to a field of detecting a watermark in a suspect image.
A known method of detecting watermarks includes the steps of pre-processing the suspect image, determining the amount of correlation between the pre-processed image and the watermark, and generating a watermark detection signal depending on the amount of correlation. The invention also relates to a device for performing the method.
The method of detecting a watermark described above is disclosed in International Patent Application WO-A 98/03014. As described in this Application, conventional watermark detection is based on determining the amount of correlation of a suspect image with the watermark to be detected. If the amount of correlation is less than a given threshold, the watermark is said to be absent, otherwise it is said to be present. The larger the correlation, the more reliable the detection and the more processing of the watermarked image is allowed until the watermark cannot be detected anymore.
To improve the reliability of the watermark detection, the suspect image is pre-processed. In the know a watermark detection method, the pre-processing includes predictive or matched filtering of the suspect image prior to the conventional watermark detection. The processed image is more correlated with the watermark than the suspect image. Such a matched filter is a convolution filter, i.e. the filtered image is a spatially-invariant linear combination of global displacements of the same image. It is optimized to attenuate the frequencies for which the image dominates over the watermark. It works equally on the watermark and the underlying image.
The above citations are hereby incorporated in whole by reference.
It is an object of the invention to provide an improved method of detecting the watermark.
In the method of the invention, the pre-processing includes the steps of subjecting the suspect image to a non-convolution filter operation so as to obtain a distorted but resembling image which has less correlation with the watermark, and subtracting the distorted image from the suspect image.
A non-convolution filter is herein understood to mean a non-linear filter or a partially-variant linear filter.
The invention is based on the recognition that most watermarking schemes are of resistant to (even minor) non-linear or spatially variant distortions. Such distortions do not necessarily affect the perceptual appearance of the image, but destroy the watermark information. The distorted image resembles the original image but has less correlation with the watermark. This property is here exploited to improve the reliability of the watermark detection. By subtracting the distorted image from the suspect image, a difference image is obtained in which the ratio between the watermark information and the underlying residual image content is greatly enhanced. The performance of the subsequent conventional watermark detector is therefore considerably improved.
Examples of non-linear filters are median filters, local minimum filters and local maximum filters. Examples of spatially variant operations are stretching, shrinking, shearing, rotation, motion compensation and region-based shifting. Various advantageous embodiments are defined in the appended claims and disclosed in the following description.